Method of evaluating semiconductor device

ABSTRACT

The present invention provides a semiconductor device having an active region bent at right angles, wherein an interval between patterns for the active region and a gate is set larger than an arc radius of a curved portion (portion where a line is brought to arcuate form) formed inside the pattern for the bent active region. By defining and designing the pattern interval, the curved portion of the active region do not overlap the gate pattern, and the difference between a device characteristic and a designed value can be prevented from increasing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transistor integrated circuit formedon a wafer, and particularly to a semiconductor device capable ofobtaining a characteristic equivalent to a design value where an activeregion or a gate of a MOS transistor is a bent pattern, and a method ofevaluating the semiconductor device.

2. Description of the Related Art

With the objective of speeding up a device, an SOI (Silicon OnInsulator) substrate in which a thin silicon layer is formed on aninsulating layer, has been in heavy usage in recent years. A method offorming a transistor on an SOI is basically similar to a process using abulk silicon wafer.

When device isolation is carried out by a LOCOS (Local Oxidation OfSilicon) method at this time, the edges of an SOI layer provided on aburied oxide film 2 are brought to bird beaks due to a LOCOS oxide film1 as shown in FIG. 4A. When a gate oxide film 4 is formed under suchshapes and a gate 5 is formed, a parasitic transistor 7 is formed ateach edge portion of an active region 3 where the gate 5 overlaps.Therefore, a transistor to be formed takes a configuration in which theparasitic transistor 7 is connected in parallel with a transistor 6 ofthe present device.

Since the impurity concentration of each bird beak is reduced due to ionimplantation conditions, annealing in each process steps, etc., theparasitic transistor is easy to be reduced in threshold voltage ascompared with the transistor of the present device. Therefore, as shownin FIG. 4B, a hump phenomenon in which humps appear, occurs in thecharacteristic of a drain current Id relative to a gate voltage Vg. Inorder to suppress the hump phenomenon, for example, ion implantation isnormally effected on the bird beaks, thereby taking or carrying outcountermeasures to raise a channel concentration of a parasitictransistor region and suppress the starting up or raising of theparasitic transistor.

Upon device's circuit design, a spice parameter extracted from atransistor characteristic of an evaluating TEG (Test Element Group) isused to carry out the circuit design. As shown in FIG. 3A, a transistorhaving simple pattern shapes in which a gate 11 and an active region 12intersect at right angles, is used in TEG at the extraction of theparameter.

On the other hand, however, each of patterns for an active region and agate in an actual circuit pattern cannot be configured in a simple shapelike a TEG pattern. In the case of, for example, an inverter circuit orthe like, it assumes a bent pattern to connect two adjacent activeregions by one gate pattern.

With a view toward making a pattern layout in consideration of highintegration such that elemental devices can be formed on a wafer as manyas possible, such a pattern layout that a gate 13 is formed on an activeregion 14 having a bent shape as shown in FIG. 3B is also used.

Now, a patent document (Japanese Unexamined Patent Publication No. Hei10(1998)-93101) has disclosed a method of providing an impurity regionhigher in concentration than an active region between the active regionand an insulating layer to suppress a hump characteristic, therebypreventing concentration of an electric field from occurring due toetching of an insulating film at an edge portion adjacent to the activeregion. Further, patent documents (Japanese Unexamined PatentPublication Nos. 2002-9292 and 2003-86807) respectively have disclosed amethod of performing impurity injection for suppressing turning on of aparasitic transistor on each side portion of an active region withoutmaking use of a mask using a photoresist to thereby prevent a humpcharacteristic.

On the other hand, when patterns for an active region having a bentshape and a gate are formed using photography, the interior of a bentportion of a photoresist's pattern do not assume the right angle as inthe photomask's pattern and is curved as indicated by a portion B inFIG. 3B. When, for example, the active region is formed whilemaintaining the curved pattern as it is, a curved portion overlaps thegate pattern where the pattern interval between the bent active regionand the gate is short. This results in the same as that a channel widthbecame longer than a design value, and hence a transistor characteristicvaries.

A section taken along line A-A of FIG. 3B is shown in FIG. 3C. An activeregion 14 provided on a buried oxide film 15 is isolated by a LOCOSoxide film 16, and a gate 13 is formed on a gate oxide film 17. In atransistor having an SOI structure in particular, the edge portion of anactive region based on a LOCOS oxide film assumes a bird beak shape whenthe curved active region overlaps a gate's pattern. Therefore, aparasitic transistor region becomes wide and hence a hump phenomenon ofan Id-Vg characteristic is exhibited greater.

Under such circumstances, the difference in characteristic becomes largebetween a transistor characteristic of an actual device and a spiceparameter used in circuit design introduced from TEG so that anintegrated circuit cannot obtain characteristics such as shown incircuit design. When a curved active region overlaps a gate pattern evenin the case of the implantation of an impurity into an edge region,which is carried out for hump suppression where a transistor is formedin an SOI substrate, the effect of the impurity injection is notobtained and hence a hump characteristic is not improved.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of such problems.It is an object of the present invention to provide a semiconductordevice capable of, when a pattern for an active region or a gate isbent, preventing a curved portion of the bent pattern from influencing adevice characteristic and reducing a characteristic difference betweenthe device characteristic and a design value, and a method of evaluatingthe semiconductor device.

According to one aspect of the present invention, for achieving theabove object, there is provided a semiconductor device comprising anactive region bent at right angles, and a gate formed over the activeregion, wherein an interval between patterns for the bent active regionand the gate is larger than an arc radius (corresponding to the radiusof an arc when a line is regarded as the arc) of a curved portion(corresponding to a portion where the line becomes arcuate) formedinside the pattern for the bent active region. Here, the term “at rightangles” does not mean an accurate angle of 90° but means that a patternis bent substantially at right angles (bent approximately at rightangles). Also, similarly, the term “arcuate” does not mean the arc of anaccurate circle but means that the line is handled as a substantiallycircular arc.

When a portion bent substantially at right angles exists in a maskpattern where a pattern for a photoresist is formed in aphotolithography process at the formation of a device pattern, its bentinner line does not reach the right angle as defined by the mask butassumes a bent round line. The present invention aims to set a patterninterval between the active region and the gate larger than the arcradius of the portion brought to such an arcuate line thereby to obtainan effect that does not influence the characteristic.

In an integrated circuit of MOS transistors, a pattern for an activeregion or a gate is often bent to enhance integration density. When, forexample, a curved portion of an active region pattern overlaps a gatepattern, it results in the same as that a channel width expands. Thus, adevice characteristic changes into a design value. Therefore, it isdesirable to define and design such an interval that the curved portiondoes not influence the gate pattern.

When a MOS transistor is formed in an SOI substrate in which deviceisolation is performed by using a LOCOS method, a large characteristicdifference is apt to occur between a device characteristic and a designvalue due to a parasitic transistor formed at a curved portion.Therefore, the interval between the patterns for the active region andthe gate is set so as not to overlap the curved portion of the activeregion, thereby bringing about the great effect of resolving thedifference in characteristic between the design value and an actuallyfabricated device. Needless to say, an advantageous effect is obtainedeven with respect to the application of a bulk structure as well as anSOI structure to a transistor.

Further, even in the case of a semiconductor device in which a gatepattern is bent at right angles, a pattern interval between the bentgate pattern and an active region is set larger than an arc radius of acurved portion lying inside the bent gate pattern, thereby making itpossible to prevent the influence of the gate's curved portion on theactive region and obtain a characteristic substantially equivalent to adesigned one.

In order to evaluate the characteristic of the semiconductor devicehaving a predetermined pattern interval between such a bent activeregion and a gate as described above, there is provided a method ofevaluating the semiconductor device comprising the steps of forming aplurality of MOS transistors different only in the pattern interval,measuring characteristics of the plurality of MOS transistorsrespectively, and plotting the characteristics relative to the patterninterval and defining such an interval that the characteristics do notdepend on the pattern interval as a minimum pattern interval between thebent active region and the gate.

When the active region is bent substantially at right angles and apredetermined interval is defined between patterns for the active regionand the gate as viewed in the direction normal to its bent direction,the semiconductor device can be evaluated by measuring a thresholdvoltage and a drain current. When the values of the threshold voltageand the drain current relative to the pattern interval between theactive region and the gate are plotted in graph form, the characteristicvaries according to the interval when the interval is small, whereas thecharacteristic becomes constant when the interval becomes large. Thatis, such an interval that the threshold voltage and the drain currentbecome constant without depending on the interval, results in theminimum interval to allow the active region pattern and the gate patternto be spaced away from each other.

Aside from the threshold voltage and the drain current, a leak currentand/or an S value (indicative of a drain current characteristic relativeto a gate voltage at less than or equal to the threshold voltage andcorresponding to a gate voltage value for incrementing the drain currentby one digit) are measured. Similarly, the dependence of thecharacteristic on the interval between the active region and the gate isdetermined, thereby making it possible to determine the interval to beaccurately taken for separation. This evaluating pattern may be insertedinto part of a wafer as TEG or may be formed over the entire surface asan evaluating wafer.

According to the present invention as described above in detail, when anactive region or a gate is bent approximately at right angles, theinterval between patterns for the active region and the gate is setlarger than an arc radius of a substantially arcuate curved portionlying inside the pattern for the bent active region, thereby making itpossible to lessen the influence of the curved portion on a devicecharacteristic and obtain a device characteristic set as substantiallydesigned. Therefore, it is possible to lessen feedback from trialproduction to design and reduce the number of man hours and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects and featuresof the invention and further objects, features and advantages thereofwill be better understood from the following description taken inconnection with the accompanying drawings in which:

FIG. 1 is a view for describing a semiconductor device according to anembodiment of the present invention;

FIG. 2 is a characteristic diagram showing characteristics (Vt, Ids,Ioff and S values) relative to an interval between an active region anda gate of the semiconductor device according to the embodiment of thepresent invention;

FIG. 3A is an explanatory view showing general patterns of an activeregion and a gate;

FIG. 3B is an explanatory view showing a pattern having a bent portionin an active region;

FIG. 3C is an explanatory view illustrating a section taken along lineA-A of FIG. 3B;

FIG. 4A is a schematic cross-sectional view showing an active region ofa MOS transistor formed in an SOI substrate using a LOCOS method; and

FIG. 4B is an explanatory view illustrating a hump characteristicrelated to drain current vs gate voltage characteristics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be explainedhereinafter in detail with reference to the accompanying drawings.Incidentally, elements each having substantially the same functionalconstitution are given like reference numerals through thisspecification and figures, and dual explanations of the same elementswill therefore be avoided.

A description will be made of a case in which an active region 100 of aMOS transistor such as shown in FIG. 1 as the present embodiment is bentat right angles. The present embodiment is characterized in that aninterval L between the active region 100 and a gate 200 is made largerthan an arc radius RL at an arcuate curved portion R of the activeregion. Here, the term “at right angles” does not mean an accurate angleof 90° but a pattern bent substantially at right angles. Also the term“arcuate” does not mean a line that accurately overlaps the arc of acircle but means that the line can be handled as a circular arc.

When circuits are intended to be integrated on a chip as many aspossible, it is advantageous to set the interval L between the activeregion 100 and the gate 200 as short as possible. Design criteria forthe interval between the active region and the gate can be determinedbased on alignment accuracy of photography. Allowance for each of anactive region and a gate of a transistor fabricated by a 0.2 μm designrule is about 0.1 μm.

However, when the active region is formed using a mask pattern bent atright angles, the interior of a photoresist's pattern is curved (curvedportion R) and a pattern line thereof assumes a substantially arcuateform. Since the active region 100 having a curved form is formed as itis, the curved portion R overlaps the gate 200 as the interval L betweenthe patterns for the active region 100 and the gate 200 becomes short(see FIG. 3B). This acts in the same manner as when a channel widthexpands. Thus, a device characteristic changes into a designed value.

Thus, the present embodiment aims to regard the substantially arcuateline of the curved portion as a circular art with its center as a pointP, set the interval L larger than the arc radius RL so as to avoidoverlapping of the curved portion R with the pattern for the gate 200and thereby prevent a variation in characteristic. Such a minimuminterval L as not to influence transistor characteristics is evaluated,and circuit design and pattern design can be performed with theprovision of rules so that the device characteristic is set as designed,thereby making it possible to lessen feedback such as a correction tothe designed value and reduce the number of man-hours and cost requiredto develop a device.

Even when the gate has a bent portion outside the active region, thepattern for the curved gate acts so as to expand a gate length, therebyvarying the characteristic. Therefore, the interval between the patternsfor the gate and active region is made larger than the arc radius of thecurved portion in such a manner that the curved portion of the gate doesnot overlap the pattern for the active region, thereby making itpossible to obtain the characteristic set as substantially designed.

In order to define the interval L between the active region 100 and thegate 200, a plurality of MOS transistors different only in the intervalL corresponding to a predetermined interval are first formed. Thesetransistors are used for evaluation but fabricated under the conditionssimilar to those for the present device. Patterns for the evaluation maybe formed in an evaluating wafer but may be formed in part of a waferfor the present device as TEG patterns.

Next, the characteristics of the formed plural MOS transistors arerespectively measured. When the result of measurement of theircharacteristics is plotted in the form of a graph with respect to theinterval L, the characteristics do not depend on the interval and aconstant interval can be provided as the minimum pattern intervalbetween the active region and the gate.

When such an active region as shown in FIG. 1 is being bent, the minimumpattern interval can be defined by measuring a threshold voltage Vt anda drain current Id as the characteristics. Further, a leak current Ioffand an S value (gate voltage value for incrementing the drain current byone digit) may be measured aside from the threshold voltage Vt and thedrain current Id, whereby the minimum pattern interval can be definedwith a higher degree of precision.

FIG. 2 shows the results of plotting of a threshold voltage Vt, a draincurrent Id, a leak current Ioff and an S value with respect to aninterval L. As evaluating devices used in measurement, NMOS transistorseach having a gate length of 0.2 μm and a gate width of 0.4 μm areformed on an SOI substrate, and transistors set at an interval L rangingfrom about 0.1 μm to 1 μm are measured.

When the active region is being bent even if the gate length and thewidth (gate width) of the active region are of the same designed valueas shown in FIG. 2, the transistor characteristics greatly vary when theinterval L is small. When the interval L reaches about 0.4 m or more,the threshold voltage Vt and the drain current Ids do not depend on theinterval L (characteristic line becomes horizontal) and hence thecharacteristic become constant.

In this case, the interval of about 0.4 m can be judged to be theminimum pattern interval that does not influence the transistorcharacteristics. That is, it is understood that as to the devicedesigned under the 0.2 μm design rule, variations in the transistorcharacteristics are less reduced if the interval between the activeregion and the gate is set to about 0.4 μm or more.

Thus, the transistor in which the pattern for the active region or gateis bent, is capable of setting the interval between the active regionand the gate to such an interval as not to be affected by the curvedportion of the bent pattern and reducing the difference between each ofspice parameters high in accuracy and an actual circuit characteristic.

Since a parasitic transistor is formed at the end of an active regionparticularly when MOS transistors are formed on an SOI wafer using aLOCOS method to fabricate a semiconductor integrated circuit, it isgreatly subject to a bent portion of the active region. However, theinterval between the gate and the active region is set to a suitableinterval to thereby allow the difference between a transistorcharacteristic and a spice parameter to be reduced more effectively.

While the preferred embodiment of the present invention has beendescribed with reference to the accompanying drawings, it is needless tosay that the present invention is not limited to the embodiment. It willbe apparent to those skilled in the art that various changes andmodifications can be supposed to be made to the invention within thescope described in the following claims. It is to be understood thatthose changes and modifications fall within the technical scope of thepresent invention.

Although the present embodiment has explained the transistor formed onthe SOI wafer, a similar advantageous effect can be expected even in thecase where a transistor is formed on an SOS (Silicon On Sapphire)substrate in which a silicon layer is formed on a sapphire substrateused as an insulator, or a normal bulk substrate.

The present invention is applicable to a semiconductor device and amethod of evaluating the semiconductor device, and particularly to asemiconductor device capable of obtaining a characteristic with nodifference from a design value in a MOS transistor having such a patternthat an active region or a gate is bent, and a method of evaluating thesemiconductor device.

1-6. (canceled)
 7. A method of evaluating a semiconductor device whereina gate is formed over an active region and the active region is bent atright angles and has a predetermined pattern interval defined away fromthe gate, said method comprising the steps of: forming a plurality ofMOS transistors different only in the pattern interval; measuringcharacteristics of the plurality of MOS transistors respectively; andplotting the characteristics relative to the pattern interval anddefining such an interval that the characteristics undepend on thepattern interval as a minimum pattern interval defined between the bentactive region and the gate.
 8. The method according to claim 7, whereinthe measured characteristics are a threshold voltage and a draincurrent.
 9. The method according to claim 8, wherein a leak current isfurther measured in addition to the threshold voltage and the draincurrent.
 10. The method according to claim 9, wherein a drain currentcharacteristic relative to a gate voltage less than or equal to thethreshold voltage is further measured in addition to the thresholdvoltage and the drain current.